Hitherto, a polypropylene resin has been well known as a general-purpose polymeric material having excellent heat resistance. However, it is known that when the resin is used for blow molding, vacuum forming, pressure forming, film extrusion, extrusion coating, spinning, and foam processing, the problems occur of non-uniformity in the wall thickness, neck-in, surging, resonance, non-uniformity in the cellular structure, etc. Many of these inferior molding problems are related with elongational viscosity and it is generally known that when the elongational viscosity is sufficiently larger than shear viscosity at the same temperature and the same rate of strain, the moldability is improved (M. Shinohara, Journal of the Society of Rheology, Japan, Vol. 19, 118 (1991)). However, in general the elongational viscosity of a polypropylene resin is only about three times the shear viscosity at the same rate of strain.
Accordingly, to increase the elongational viscosity of a polypropylene resin, a method of broadening the molecular weight distribution of a polypropylene resin or adding a branched type low density polyethylene (LDPE) has been conducted.
Furthermore, in recent years polypropylene having long chain branches has been developed and is on the market (Plastic Engineering, page 82, March 1991).
However, the limit of the ratio of elongational viscosity/shear viscosity of a polypropylene resin at the same temperature and the same rate of strain is about 4, and the improved effect on the processability is insufficient. On the other hand, the polypropylene having long chain branches has poor melt drawing property, so that poor molding frequently occurs.